Index read view supports the `quantile` method now

Since: 3.6.0

Now quantile method (tarantool/doc#4975) can be used in read view:

rv = box.read_view.open()
rv.space.test.index.pk:quantile(0.5, {10}, {20})

The method also works with read view spaces:

rv.space.test:quantile(0.5, {10}, {20})

Also, raw read views (ones that can be used from C API) are popualted
with this method as well:

API_EXPORT int
box_raw_read_view_quantile(
	const box_raw_read_view_index_t *index, double level,
	const char *begin_key, const char *begin_key_end, const char *end_key,
	const char *end_key_end, const char **quantile_key,
	const char **quantile_key_end);

Note that one needs to call box_region_truncate after calling
raw read view quantile sine the result (the quantile key)
is allocated on region.
Requested by @drewdzzz in https://github.com/tarantool/tarantool-ee/commit/d4c7384e341131a9f55b06c3b67efdc0b25b7348.

Search the read view documentation for the existing quantile method and the raw read view API entry points shown in the issue. Document the Lua index and space examples, the C API signature, and the need to call box_region_truncate after the raw result; done means all three usage paths and the region-lifetime note are covered.